Why Uncaught TypeError: Cannot read property 'Constructor' of undefined?

Question

JavaScript

There is a modal on the page that opens a form with two textarea:

<div class="modal box-modal">
<form class="">
    <div class="form_row">
        <div class="input_cover">
            <textarea class="ckeditor" rows="10" cols="80" name="editor1" id="editor1"></textarea>
        </div>
    </div>
    <div class="form_row">
        <div class="input_cover">
            <textarea class="ckeditor" rows="10" cols="80" name="editor2" id="editor2"></textarea>
        </div>
    </div>
    <button class="save">Сохранить</button>
</form>
</div>

There was a problem that it opened with two editors, but they could not be edited.
There are a lot of people on the Internet who solved the problem, let's go with the code:

<script>
    $(document).ready(function () {
        $.fn.modal.Constructor.prototype.enforceFocus = function () {
            modal_this = this;
            $(document).on('focusin.modal',
                function(e) {
                    if (modal_this.$element[0] !== e.target &&
                        !modal_this.$element.has(e.target).length
                        // add whatever conditions you need here:
                        &&
                        !$(e.target.parentNode).hasClass('cke_dialog_ui_input_select') &&
                        !$(e.target.parentNode).hasClass('cke_dialog_ui_input_text')) {
                        modal_this.$element.focus();
                    }
                });
        };
});
</script>

But when loading the modal, they are still not available, and an error from the topic topic also appears.

It's funny that I inserted the same code into fiddle and other analogues - everything worked ...

What could be wrong?